Lithium primary batteries have been widely used in devices having a temperature of operating environment of about −20° C. to 60° C. based on human life range. In recent years, uses of devices using primary batteries have been increasing. In particular, primary batteries having high output have been demanded as the devices are having higher functions and becoming lightweight. For example, in active tags or keyless entry, a large current is required at the time of transmitting radio waves. In particular, primary batteries operating favorably in a low-temperature environment of about −20° C. have been demanded.
Lithium primary batteries include: a positive electrode including a positive electrode active material of a metal oxide such as manganese dioxide (MnO2), graphite fluoride ((CFx)n), iron sulfide (FeS2), and thionyl chloride (SOCl2); a negative electrode including lithium or a lithium alloy; a separator; and a non-aqueous electrolyte. In particular, lithium primary batteries using manganese dioxide as the positive electrode active material are widely known because of having favorable discharge characteristics, and moreover, manganese dioxide is a material that can be procured relatively easily.
On a surface of the negative electrode, active lithium reacts with components included in the non-aqueous electrolyte, thereby producing a gas or forming a coating film of a high-resistance (or insulating) component. Consequently, the battery reaction may be impaired, or the resistance of the battery may be increased, which may deteriorate discharge characteristics.
In a lithium primary battery using manganese dioxide or graphite fluoride as the positive electrode active material, a part of the active material dissolves in the non-aqueous electrolyte to produce manganese ions or fluorine ions in the battery. The resulting ions produce a high-resistance component by the reaction with the negative electrode, and a coating film of this component is formed on the surface of the negative electrode, which leads to an increase in the resistance of the battery.
In particular, when manganese dioxide is used as the positive electrode active material, the elution of manganese ions increases as the discharge progresses. Consequently, if the battery is stored in the condition of being discharged to some extent, a large amount of high-resistance component is produced on the surface of the negative electrode, which increases significantly the resistance of the negative electrode and the battery. Thus, when the use of the battery is resumed, the discharge characteristics decline significantly although the capacity of the battery remains. In particular, the large-current discharge characteristics and the pulse discharge characteristics at low temperatures decline considerably, and therefore the battery will not be able to be discharged at a large current.
In order to suppress the elution of the positive electrode active material and protect the surface of the negative electrode to suppress the formation of the high-resistance coating film, an additive to the non-aqueous electrolyte has been examined. However, the increase in the resistance on the surface of the negative electrode cannot be suppressed sufficiently, and therefore the realization of higher output in a low-temperature environment has not been satisfactory.
In order to suppress the reaction of the negative electrode with the components included in the non-aqueous electrolyte on the surface of the negative electrode, an arrangement of a layer using a carbon material on the surface of the negative electrode has been examined.
For example, in Patent Literature 1, in order to prevent the reaction between lithium and the electrolyte and to suppress the increase of inactive lithium, a carbonaceous powder adheres to a surface of metal lithium or a lithium alloy and is pressed with rollers to be integrated with the metal lithium or lithium alloy.
In Patent Literature 2, in order to suppress the reaction of the negative electrode with the electrolyte, a carbon material-including layer formed of a carbon material and a material holding the carbon material is disposed on the surface of the negative electrode.
In Patent Literature 3 relating to a secondary battery, a thin layer of a carbon material composed of an organic sintered compact is formed on a surface of the negative electrode by application, vapor deposition, or sputtering, etc. Thus, suppression of the production of lithium dendrites on the surface of the negative electrode is intended, thereby to improve the charge and discharge cycle life and the storage characteristics.